1. Field of the Invention
This invention relates to optical fibers. More specifically, this invention relates to optical fibers having relatively low indices of refraction.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
In conventional fiber optic sensors light guided within an optical fiber is modified in reaction to various external physical, chemical or similar influences. In operation, light from a source having relatively stable optical properties is typically coupled into the fiber of the conventional sensor. The light is then directed by the fiber to a region in which a measurement is to take place. In extrinsic sensors the guided light then may exit the fiber and interact with the substance being measured (measurand) prior to again being launched into the same or a different fiber. Alternatively, in intrinsic sensors the light remains within the fiber throughout the measurement region.
In extrinsic sensors reflection losses are incurred as the guided light leaves the optical fiber and enters the measurand. In particular, these reflection losses are proportional to the difference between the indices of refraction of the fiber and the measurand. At present, the minimum index of refraction (at visible wavelengths) of optical fibers is believed to be approximately 1.4. Consequently, reflection losses are generally unavoidable when extrinsic sensors are employed to analyze material having an index of refraction (n) less than 1.4. For example, in the visible spectrum water exhibits an index of refraction of approximately 1.33.
In another type of optical fiber sensor, generally known as an evanescent field sensor, the light guided by the fiber partially couples to the measurand via an evanescent (i.e. exponentially decaying) field which surrounds the fiber. That is, a portion of the optical energy carried by the fiber (the evanescent wave) propagates along the length of the fiber within a region of space immediately surrounding the cladding. The measurand surrounding the fiber may either absorb or change the properties of the evanescent field, thus enabling a measurement to be performed. Employment of evanescent mode over extrinsic mode fiber sensors may be preferred in applications requiring direct interaction between the light beam and the measurand since the former approach requires no relaunching of the beam.
Unfortunately, the cladding layer of optical fibers included within conventional evanescent mode sensors is typically required to have an index of refraction less that of the measurand. In addition to water, measurands such as gasoline (n 1.4) having low indices of refraction are thus generally incompatible with evanescent wave sensors employing conventional optical fibers.
Accordingly, a need in the art exists for an optical fiber having an index of refraction less than that known to be conventionally available.